1. Field of the Invention
The present invention relates to a method for manufacturing an electrode of an electrochemical device such as a nonaqueous electrolyte secondary battery. More particularly, it relates to a technology for controlling an amount of absorbed lithium, which compensates irreversible capacity of an electrode with high accuracy and suitable for mass production.
2. Background Art
Since an electrochemical device using lithium has large theoretical capacity, it has been developed to be used as a main electric power source of various equipments. Hereinafter, a nonaqueous electrolyte secondary battery is described as an example.
It is required that a nonaqueous electrolyte secondary has larger capacity. Therefore, a negative electrode active material is being changed from a carbon material such as graphite having theoretical capacity of less than 400 mAh/g to a material containing silicon, tin, or the like, having nearly 10 times larger theoretical capacity.
All negative electrode active materials, even a carbon material, have a loss of capacity (irreversible capacity) that is difference between chargeable capacity and dischargeable capacity at initial charge and discharge. In particular, the irreversible capacity of a large capacity material containing silicon, tin, or the like, is known to be large. This irreversible capacity is thought to be generated because lithium is inactivated by the side reaction with an electrolyte solution or with a negative electrode active material at the time of charging. The irreversible capacity thus caused by a negative electrode active material may cause a loss of a part of the reversible capacity of a positive electrode that is a source of lithium ions forming the substance of battery capacity. Therefore, the irreversible capacity results in the deterioration of battery capacity.
In order to prevent the deterioration of battery capacity caused by irreversible capacity, technologies for supplying a negative electrode active material with lithium in advance have been proposed. Specifically, a method of attaching a foil of metallic lithium and the like to a negative electrode using composite oxide containing tin and the like, by, for example, a roll transfer is disclosed. Thereafter, by injecting an electrolyte solution when a battery is assembled, a negative electrode active material is allowed to absorb lithium (see, for example, International Publication No. WO 96/027910).
In this method, however, an amount of lithium that is intended to be absorbed by the negative electrode active material is much smaller than the amount of provided lithium based on the lower value of the thickness (about 30 μm) of a metallic lithium foil that can be subjected to handling. Therefore, a metallic lithium foil is placed substantially partially, and it is not possible to allow the negative electrode to absorb lithium uniformly. As a result, deformation of the negative electrode due to expansion or ununiformity of the charge and discharge reaction may be caused.
In addition, a method of forming a layer of light metal such as metallic lithium on a mixture layer containing a negative electrode active material by a dry film formation method such as a vacuum evaporation method is disclosed. When this negative electrode is held in a dry atmosphere or an electrolyte solution, lithium is absorbed by the negative electrode (see, for example, Japanese Patent Application Unexamined Publication No. 2005-038720).
In this method, it is possible to form a lithium layer itself that is thinner than a lithium foil by a vacuum evaporation method. However, even when the thickness of the lithium layer differs from the desired value for some reason, it is not possible to know the difference. Therefore, a negative electrode in which the thickness of the lithium layer significantly differs from the desired value may be produced continuously.
Additionally, a technology of producing an electrochemical cell prior to the assembly of a battery and charging a negative electrode by using a counter electrode capable of absorbing and releasing lithium is proposed (see, for example, Japanese Patent Application Unexamined Publication No. H06-325765).
In this method, it is possible to precisely evaluate an amount of lithium to be provided to a negative electrode active material from the amount of current to be passed in the electrochemical cell. However, when such an electrochemical cell is configured, continuous production becomes difficult and thus this method is not suitable for mass production.